docs: convert architectural overview diagrams from JPG to mermaid

[jasonmadigan]

Summary

• Replace the 7 static JPG diagrams in docs/design/architectural-overview-v1.md with mermaid blocks and remove the JPGs, so diagrams live in the markdown source and can be reviewed and updated as text
• Diagrams are redrawn 1:1 from the original images: node sets, exact labels, arrow directions, dashed vs solid styles, groupings and colour coding all match. The DNS zone record set for the multi-cluster layout is embedded in that diagram, as in the original image

Test evidence

• All 7 blocks render with @mermaid-js/mermaid-cli, zero errors
• Each rendered diagram compared element-by-element against its source JPG (node inventory, label text, edge directions, dashed vs solid, group membership): no discrepancies found
• Prose outside the diagram blocks is unchanged from main

How to review

The original JPGs are visible as removed files in the diff. Compare each mermaid block against its image; paste blocks into https://mermaid.live if the GitHub render is awkward to zoom.

Closes #173

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[jasonmadigan]

First pass is very inaccurate - will need more iterations